Process of cleaning and coating ferrous metal



July 13, 1950 E. R. BOLLER El AL PROCESS OF CLEANING AND COATING FERROUSMETAL Filed March 12, 1946 [N VENT? vs Ernest R. B 0 LL61", Lowell/DEuban/f? C Larence A. L Lifler BY wad 6 a/daw ATTORNEY Patented July 18,1950 PROCESS OF CLEANING AND COATING FERROUS METAL Ernest It. Boller,Marion, Ind., and Lowell D. Eubank, South Euclid, and Clarence A.Littler, Maple Heights, Ohio, assignors to E. I. du Pont de Nemours &Company, Wilmington, DeL, a

corporation of Delaware Application March 12, 1946, Serial No. 653,968

Claims. (01. 117-51) This invention relates to the coating of ferrousarticles with a zinc aluminum alloy and is more particularly directed toprocesses in which a ferrous article is first passed through a moltenzinc chloride flux which is supported upon a mass of molten zinc andthereafter passed through a molten zinc-aluminum alloy containing lessthan about 0.05 per cent by weight of iron.

Methods for galvanizing, in which ferrous articles are passed through amolten flux layer supported upon a bath of molten zinc and then throughthe zinc, are well understood, but when it is attempted to adapt suchgalvanizing technique to methods for coating ferrous articles withmolten zinc-aluminum alloys difliculties are immediately encountered.First, fluxing techniques for cleaning the metal are found to beinadequate. When zinc chloride-ammonium chloride fluxes are used on topof the molten alloy it is found that the flux rapidly becomes so viscousthat it soon loses its cleaning and fluxing ability and becomesimpossible to use. Moreover when procedures ordinarily used in zincgalvanizing are applied to zinc-aluminum galvanizing it is found thatthe molten alloy becomes badly contaminated with dross and that thealloy coating on the articles as withdrawn from the molten metal ismarred by dross. These difficulties are encountered even when thealuminum content of the bath is as low a from /2 to 1 That so small analuminum content should give rise to such pronounced difiiculties issurprising but nevertheless is the fact and the lack of a solution tothese problems has mitigated against the widespread production ofzinc-aluminum coatings even though it was recognized that such coatingshave better tightness and corrosion resistance than ordinary zinccoatings.

It is an object of this invention to provide galvanizing processes whichare simple and economical and which yield coated ferrous articles ofgood appearance and character. It is a further object to provideadherent coating of zincaluminum alloys upon ferrous articles. It is astill further object to provide a process for cleaning steel prior tocoating with a zinc-aluminum alloy. Further objects will become apparenthereinafter.

Described with reference to the drawing, the foregoing and other objectsof the invention are attained by first passing a ferrous article, ithrough a molten zinc chloride flux, 2, which is supported upon a bathof molten zinc, 3. The ferrous article is then removed from the flux,Without its being permitted to come in contact with thezinc, and ispassed into a zinc-aluminum galvanizing bath, 4, containing not morethan about 0.05per cent by weight of iron.

a By. use of the processes of the invention one is enabled economicallyand practically to use zincaluminum alloys for the coating of steel andother ferrous articles. The articles are readily cleaned at low costwithout the use of expensive cleaning agents or processes such as thoseinvolving inert atmospheres and without rapid destruction of the mainflux body.

The flux bath employed according to the present invention is comprisedof molten zinc chloride. This may additionally contain ammonium chlorideor zinc ammonium chloride which will in the course of use be convertedto zinc chloride by the underlying molten zinc layer. Other flux altsand additions may be included as desired. While the molten zinc chlorideshould be the principal constituent, the flux in a preferred embodimentof the invention desirably may contain an alkali metal chloride, or amixture of alkali metal chlorides such as sodium chloride and potassiumchloride, in an amount up to about 25 per cent by Weight. A mixture ofcommercial zinc chloride, 2.5% sodium chloride, and 2.5% potassiumchloride has given excellent results.

This molten flux i supported upon molten zinc. The zinc functions toremove iron compounds from the flux and continuously prevents build-upof iron compounds in the flux. The amount of molten zinc must besufficient with relation to the amount of flux to maintain continuouslya sufficiently low iron content. The rate of iron removal in a wellagitated bath with a fixed Zn area is a function of the concentration ofiron in the flux which, for instance, with .2a flux and a Zn surface of0.441 sq. ft. can be expressed by the empirical relationship:

t=-3.98-6.l7 log C where t time (hours) and C iron content From thisequation the rate of iron removal at any iron concentration can bedetermined. For example the removal rate at 0.1% Fe was 0.0352 lbs. Feper hour per sq. ft. of zinc surface.

It will also be understood in this connection that the rate of passageof ferrous articles through the flux must not be too slow nor the totalsurface passed per unit of time too fast with relation to the amount ofzinc and the amount of flux or the iron build-up" may exceed the rate ofremoval by the zinc. Iron dissolved in the flux from the ferrousarticles, and if the articles remain too long in the flux, the irondissolves to an undesirable degree. An ide of the rate of suchdissolution may be gained from the fact that, in a flux containing 95%commercial zinc chloride, 2.5% sodium chloride, and 2.5% potassiumchloride at 450 0., 2.05 l0- lbs. of steel per sq. ft. of surface of 24gage steel sheet was dissolved during a five second exposure.

The iron removed from the flux forms as a dross in the zinc metal and,being more dense than the zinc, sinks to the bottom. This is shown as 5in the drawing.

The zinc used for the molten sub-stratum may be composed of any of theordinary commercial grades of zinc such as Special Hi grades,Intermediate, Selected or Prime Western.

The zinc-aluminum alloy constituting the galvanizing bath should containat least about 0.2% of aluminum, and it is particularly preferred thatthe bath contain from about 0.2% to .5% of aluminum. As' in the case ofthe sub-stratum above described, the zinc used for the galvanizing bathmay be any of the commercial grades of zinc. Electrolytic zinc would bepreferred except for its somewhat higher cost. Selected zinc or PrimeWestern zinc may be used, the

latter however, only if the Fe content is below the maximum allowable0.08% or if it be mixed with a suitable quantity of Zn low in Fe. It istherefore ordinarily desirable to select a zinc with as low an ironcontent as practicable say of the order such that total iron duringcontinuous running does not build up to exceed about 0.05%.

The galvanizing bath may of course contain minor additions such as tin,lead, antimony, or cadmium in accordance with standard practicesalthough these are not generally preferred. In any event it willordinarily be preferred that the zinc content of the galvanizing bathexceed about, say, 98%.

It will be understood that iron dissolves from the ferrous objects beingcoated in the zincaluminum bath, and therefore the time of contact ofthe objects with the zinc-aluminum should be as short as feasible. In aparticular case, for example, the rate of dissolution of iron from 24gage cold-rolled sheet in a 99.5% zinc-0.5% aluminum bath was found tobe 9.0'7 10 lbs. per sq. ft. of surface for a 5 second dip.

It will be seen that by the operation of a process as above describedone is enabled quickly, economically, and efficiently to clean ferrousarticles prior to galvanizing and to provide ferrous articles whichcontain little or no iron compounds on their surfaces. This is importantfor the presence of such compounds as has heretofore been indicatedrapidly causes deterioration of a zinc-aluminum galvanizing bath byforming objectionable dross.

The invention may be better understood by reference to the followingillustrative example:

Example A quantity of 24 gage cold-rolled steel is prepared for treatingaccording to a process of this invention by pickling for five minutes in9% sulfuric acid aqueous solution inhibited with 0.05% of sulfurizedquinoidine at 80 C., for 30 seconds in hot water, dipped for one minutein a solution of 0.1% tetrasodium pyrophosphate- 0.1% soap solution at80 C. and rinsed again in hot water for 30 seconds.

The sheet, so prepared, is dipped while still wet and hot into a moltenflux comprising 95% commercial zinc chloride, 2.5% sodium chloride, and2.5% potassium chloride maintained at 450 C.

over a substratum of molten zinc with which the.

steel makes no contact.

After 5. seconds in the molten fiux, during which the sheet wascompletely cleaned of iron salts and oxides, the sheet was withdrawn,rapidly scraped free of all but a thin film of flux, and dipped whilehot in a bath of molten zinc-aluminum comprising 99.5% Grasselli Selectzinc and 0.5% aluminum at 430 C. The sheet is withdrawn in 5 seconds andexcess coating removed by shaking. The resultant coating is free ofvisible dross crystals and is exceedingly adherent as determined bytests of direct adhesion and by flexing tests where the steel could beruptured without flaking off the zinc-aluminum coating.

We claim:

1..In a process for galvanizing the steps comprising supporting a moltenzinc chloride layer upon and in contact with a molten zinc bath, passinga ferrous article into nd out of said supported zinc chloride layeronly, and then passing the ferrous article while still hot through amolten zinc-aluminum bath which does not contain more than about .05% ofiron.

2. In a process for applying a zinc-aluminum coating to a ferrousarticle the steps comprising supporting a molten zinc chloride layerupon and in contact with a molten zinc bath, passing a ferrous articleinto and out of said supported zinc chloride layer only, and thenpassing the ferrous article while still hot through a moltenzinc-aluminum bath which contains 0.2 to 1.5% aluminum and which doesnot contain more than about 0.05% of iron.

3. In a process for applying a zinc-aluminum coating to a ferrousarticle the steps comprising supporting a molten zinc chloride layercontaining an alkali metal chloride upon and in contact with a moltenzinc bath, passing a ferrous article into and out of said supported zincchloride layer only, and then passing the ferrous article while stillhot through a molten zincaluminum bath which contains 0.2 to 1.5%aluminum and which does not contain more than about 0.05% of iron.

4. In a process for cleaning a ferrous metal the steps comprisingsupporting a molten zinc chloride layer upon and in contact with amolten zinc bath, and passing the ferrous metal into and out of saidsupported zinc chloride layer only.-

5. In a process for cleaning a ferrous metal the steps comprisingsupporting a molten zinc chloride layer containing an alkali metalchlo-, ride upon and in contact with a molten zinc bath,

and passing the ferrous metal into and out of said supported zincchloride layer only.

ERNEST R. BOLLER. LOWELL D. EUBANK. CLARENCE A. LITTLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 244,153 Spear July 12, 18811,276,977 Shoemaker Aug. 27, 1918v 1,334,092 Harmeling Mar. 16, 19201,374,233 Reinhold Apr. 12, 1921. 1,722,949 Wehr July 30, 1929 1,741,388Wehr Dec. 31, 1929 1,965,759 Baldwin July 10, 1934 2,008,879 SimmonsJuly 23, 1935 2,095,198 Reid Oct. 5, 1937 2,101,553 Mattsson Dec. 7,1937

1. IN A PROCESS FOR GALVANIZING THE STEPS COMPRISING SUPPORTING A MOLTENZINC CHLORIDE LAYER UPON AND IN CONTACT WITH A MOLTEN ZINC BATH, PASSINGA FERROUS ARTICLE INTO AND OUT OF SAID SUPPORTED ZINC CHLORIDE LAYERONLY, AND THEN PASSING THE FERROUS ARTICLE WHILE STILL HOT THROUGH AMOLTEN ZINC-ALUMINIUM BATH WHICH DOES NOT CONTAIN MORE THAN ABOUT .05%OF IRON.